This invention related generally to the optical arts of holography and diffraction gratings, and, more particularly, to special forms of holograms and diffraction gratings.
Current techniques for making a master hologram or diffraction grating from which replicas can be mass-produced are well known. Briefly, and very generally, two coherent light beams are directed together at a finite angle between them at a photosensitive material surface. An interference pattern between the beams is recorded by the photosensitive material. A photoresist film is commonly used as the photosensitive material, resulting in recording the pattern of interference between the beams as a surface relief pattern.
A desired light pattern can be reconstructed from such a surface relief pattern by diffraction of a portion of illuminating light. If an object is positioned in one of the two interfering beams during exposure of the photosensitive medium, illumination of the surface relief pattern will reconstruct an image of that object in light diffracted by the surface relief pattern when illuminated. For the purposes of this description, the term "hologram" will be used to refer to any such surface relief pattern, or replicas thereof, that is formed as the result of the interference between two coherent light beams, regardless of whether light diffracted from it reconstructs an image of a recognizable object or some other desired light distribution.
Once such a master surface relief hologram is formed, the surface relief pattern is converted to a physically more rugged structure from which replicas can be made. A first step in a usual commercial process is to form a thin layer of nickel by an electrolysis process that conforms to the surface relief pattern. The nickel layer is then mounted on a sturdy support. From this metal master hologram, a number of sub-masters are usually made. Each sub-master replicate of the master surface relief pattern is used to make a large number of copy holograms.
There are two basic techniques being utilized to make hologram replicas. One is an embossing technique wherein the sub-master is urged against thin plastic film under sufficient heat and pressure to transfer the surface relief pattern into a surface of the film. The second technique is a casting process wherein a liquid resin is trapped between the surface relief pattern of a sub-master and a plastic film while the resin is hardened by actinic radiation or other curing technique. When the sub-master and film are separated, a cast surface relief pattern remains attached to the plastic film.
A next step, in both the casting and the embossing replication processes, is to coat the surface relief pattern with a thin layer of opaque, reflective material, usually aluminum. A variation in the sequence of steps for the embossing process is to coat the film with such a reflective layer prior to embossing the surface relief pattern into it. The result for any of these processes is a hologram in which the recorded light pattern is reconstructed in light diffracted in reflection from the coated surface relief pattern. The reflective hologram replica is then attached by lamination or otherwise to a substrate, such as a rigid plastic piece in the case of a credit card using such a hologram for authentication, cardboard in the case of a hologram used in product packaging, or paper in the case of a hologram used as part of a printed document.
Recently, applications have emerged wherein it is desirable that the hologram be only partially reflective so that the hologram may be mounted on a substrate over a photograph, printing and the like that is desired to be authenticated. An example is in the case of passports where such a partially reflective hologram covers at least the passport holder's photograph so that it cannot be removed or altered without destroying the authenticating hologram. The photograph or other material to be authenticated is viewable through the hologram.
One technique for doing this is to maintain a continuous metallized layer on the hologram replica but control its thickness so that it reflects only a portion of illuminating light incident upon it. U.S. patent application Ser. No. 835,524, filed March 3, 1986, of G. O. Searle, now abandoned, describes such a technique. Alternatively, a discontinuous pattern of opaque reflective material is deposited on the surface relief pattern. A hologram image is formed by reflection from the portions of the hologram carrying the reflecting material while a photograph or other material to be protected on a substrate is viewable through the hologram. The discontinuous reflective layer is described in application Ser. Nos. 156,305, filed February 12, 1988, and 160,641, filed February 26, 1988, of Donald W. Mallik. Such partially reflective holograms have also been used in product packaging and other applications where authentication of material on the substrate is not a principal purpose for using the hologram.
It is a primary object of the present invention to provide a hologram replica, combination of a hologram and a substrate, and techniques for making and using them, which are simpler, less costly and which open up a wide range of hologram applications which have not before been practical or possible.